Tires are sometimes desired with treads for promoting traction on wet surfaces. Various rubber compositions may be proposed for such tire treads.
For example, tire tread rubber compositions which contain high molecular weight, high Tg (glass transition temperature) diene based elastomer(s) might be desired for such purpose particularly for wet traction (traction of tire treads on wet road surfaces). Such tire tread may be desired where its reinforcing filler is primarily precipitated silica which may therefore be considered as being precipitated silica rich.
In one embodiment, the improved predictive wet traction performance for the tread rubber composition is based on a maximization of its tan delta physical property at about 0° C. and a desired low value for rebound at about 0° C.
However, it might also be desired to provide such tread rubber composition containing a high Tg styrene/butadiene elastomer (SBR) for wet traction with a lower stiffness at lower temperatures to promote cold weather winter performance, particularly for vehicular snow driving.
In one embodiment, the predictive cold weather performance for the tread rubber composition is based on a minimization of its stiffness physical property at about −20° C. (e.g. minimized storage modulus G′).
Therefore, it is desirable to provide such vehicular tire tread with a rubber composition containing high Tg SBR elastomers with an optimized (maximized) tan delta property at about 0° C. (for predictive wet traction performance improvement) combined with an optimized (minimized) stiffness property at about −20° C. (for predictive cold weather performance improvement).
It is considered that significant challenges are presented for providing such tire tread rubber compositions that provide a combination of both wet traction and winter performance. To achieve the challenge of providing such balance of tread rubber performances with tread rubber compositions, it is recognized that concessions and adjustments would be expected. To meet such challenge, it is desired to evaluate rubber compositions with a combination of:
(A) utilizing a high Tg styrene/butadiene rubber,
(B) utilizing a low Tg cis 1,4-polybutadiene rubber,
(C) providing reinforcing filler containing primarily chemically pretreated precipitated silica (CTS), (chemically pretreated prior to its addition to the rubber composition, particularly pre-hydrophobated), and a minor amount of rubber reinforcing carbon black to also promote wet traction for the tire tread rubber composition, (in one embodiment, the filler reinforcement is comprised of from about 60 to about 99 percent of the CTS filler and the rubber composition is sometimes referred to as being precipitated silica rich),
(D) providing polyethylene glycol to promote maintaining a higher stiffness of the rubber composition at the operating temperature range of the tire tread, particularly in a range of about 40° C. to about 60° C., particularly polyethylene glycol having an average molecular weight in a range of from about 2000 to about 6000, and
(E) optionally providing a traction promoting resin.
Such CTS is provided as a precipitated silica pretreated with a silica coupler such as a bis(3-triethoxysilylpropyl) polysulfide or alkoxyorganomercaptosilane, particularly an alkoxyorganomercaptosilane.
Such rubber compositions may contain a petroleum and/or vegetable triglyceride based rubber processing oil to reduce the viscosity of the uncured rubber composition and to thereby promote more desirable processing conditions for the uncured rubber composition. In practice, a high viscosity SBR may be extended with the petroleum based oil or vegetable triglyceride oil in a sense of adding the oil to a polymerization cement containing the high viscosity SBR (e.g. high Mooney 1+4, 100° C. viscosity) following polymerization of styrene and 1,3-butadiene monomers to form a composite of oil extended SBR with the petroleum oil or vegetable triglyceride oil before the composite is added to the rubber composition in an internal rubber mixer (e.g. Banbury rubber mixer). Alternately, the petroleum based oil or vegetable triglyceride oil may be added to the rubber composition in an internal rubber mixer to reduce the viscosity of the uncured rubber composition both in the internal rubber mixer and for subsequent rubber processing in a rubber processing apparatus such as, for example, in a rubber extruder.
As indicated, it is considered that significant challenges are presented for providing such tire tread rubber compositions for maintaining a balance of wet traction, winter performance, rolling resistance and treadwear properties.
To achieve such balance of tread rubber performances with tread rubber compositions containing a combination of high Tg styrene/butadiene rubber (SBR) and low Tg cis 1,4-polybutadene rubber (PBd) based elastomers together with pretreated precipitated silica (CTS) is used to promote a reduction in the stiffness of the cured rubber composition over a wide temperature range, which is good for obtaining low stiffness at low temperatures for winter performance when used with combinations of petroleum based rubber processing oils and/or vegetable triglyceride oils, and also for obtaining a combination of good wet traction and beneficially reduced rolling resistance for a tire with tread of such rubber composition.
Such vegetable oils are generally contemplated as at least one of soybean, sunflower and rapeseed oils, particularly soybean oil.
However, while the use of the CTS has been observed to promote a desirable lower G′ stiffness at about a −20° C. low temperature for the cured rubber composition, it has also been observed to promote a corresponding reduction in stiffness of the cured rubber composition at a higher intended operating temperature range (about 40° C. to about 60° C. or an even higher temperature) of the tire. Such loss of stiffness at such higher operating temperature for a tire tread rubber composition can lead to a reduction of wet and dry handling performance for the tire. The CTS in such application has also shown a loss of laboratory determined abrasion resistance, which would be predictive of worse resistance to tread wear for tire treads of such rubber compositions.
To meet such challenge of providing a precipitated silica-rich tread rubber composition containing high Tg SBR to promote wet traction and low Tg PBd to promote treadwear performance combined with promoting a reduction in its stiffness at low temperatures, but maintaining stiffness at higher operating temperature conditions, it is desired to evaluate the following approach:
(A) replacing a portion of the rubber processing oil (e.g. petroleum based oil and/or vegetable triglyceride oil) with polyethylene glycol to promote a suitable uncured rubber processing viscosity and to promote a lower cured stiffness of the tread rubber composition at lower temperatures to thereby promote low temperature winter performance for the rubber composition, while maintaining the desired stiffness at higher operating conditions for the rubber composition to thereby promote maintenance of wet and dry handling performance for the tire,
(B) providing a high content of CTS precipitated silica-rich rubber reinforcing filler to promote wet traction for the cured rubber composition.
In the description of this invention, the terms “compounded” rubber compositions and “compounds” are used to refer to rubber compositions which have been compounded, or blended, with appropriate rubber compounding ingredients. The terms “rubber” and “elastomer” may be used interchangeably unless otherwise indicated. The amounts of materials are usually expressed in parts of material per 100 parts of rubber by weight (phr).
The glass transition temperature (Tg) of the solid elastomers may be determined by DSC (differential scanning calorimetry) measurements, as would be understood and well known by one having skill in such art. The softening point of a resin may be determined by ASTM E28 which might sometimes be referred to as a ring and ball softening point.